I am user:Gymel on the German Wikipedia. Over there my involvement with wikipedia started in early 2010 with the then new idea of BEACON files (see meta:BEACON for an english description) which boils down to slightly formalize simple lists of identifiers to faciliate exchange of "interesting" links between Wikipedia, library catalogues, biographical dictionaries, repositories of source materials and so on. As it became clear at that time, the Personennamendatei (PND) originally created as a name authority file for library consortia in Germany and Austria had been in heavy use by several bibliographies (not surprising) but also for the important Allgemeine Deutsche Biographie and Neue Deutsche Biographie, several prosopographies, manuscripts and letters, and for indexing in the context of editions. By consumption of the standardized BEACON files (most of them are published, web accessible and updated on a regular base) the respective web sites could easily interlink their biographical resources obliviating the former need to negotiate mutual "exchange" of this kind of data, and analyze and process as many different formats as there were suppliers. It also turned out that volunteers can easily create and publish unauthorized "external" BEACON files, especially when the indexed resources are comparatively stable like in the case of historical encyclopedias.
In 2010 de:Wikipedia and de:Wikisource had already been using PND identifiers for several years: This originally started by the desire to link from Wikipedia articles to the literature accounted for in the online catalogue of the Deutsche Nationalbibliothek (DNB, german national library) and had already lead to two important practices: DNB on a daily base imports data provided by de:Wikipedia enabling their online catalogue (which also presents authority data) to link to the corresponding wikipedia articles. And DNB commited itself to process all correction requests to their (personal) authority records backed by information from wikipedia articles when submitted on de:Wikipedia:PND/Fehlermeldung. And there of course was the "Wikipedia Personenensuche" on the toolserver http://toolserver.org/~apper/pd/ which still is in search of a better name: Mashing up data extracted from wiki links and templates in de:Wikipedia, de:Wikisource and Wikimedia commons with other information on the web (open library data, matching links by virtue of BEACON files) by means of identifiers and being accessible by several of these identifiers this tool in many aspects is somehow comparable to VIAF: Altough "searching" is possible the main application lies in targeted access of something you already know it is there and then "discovering" additional resources. With this base of operation the page de:Wikipedia:BEACON quite naturally established itself as place for the formulation of the minimal BEACON standard and as initial publishing platform for the emerging BEACON files.
Originally PND numbers were kept in de:Template:PND which served to produce visible links in the weblinks section of biographical articles. Recognizing the improving situation for detection of identifiers for other authority data schemes namely Library of Congress Name Authority File (LoC-NA or LCNAF) and VIAF since they became online and actually disclose data helping to identify and distinguish persons de:Template:Normdaten was introduced in 2009, probably chiefly motivated by the perspective of uniformly adding more identifiers without the need to craft more templates. Identifiers in the Normdaten template without "corresponding" library data and later the wealth of BEACON files exemplifying what can be done with authority data outside the narrow realm of "give access to printed books" helped to understand the dual nature of authority data and the quandary of the old PND template: Consequently the task of identifying was delegated to the Normdaten template and for the purpose linking to notices or giving access to materials de:Template:DNB-Portal was introduced in late 2009 to replace the PND template. (There were aberrations from this development, since you can link to WorldCat with LCNAF identifiers the display of the Normdaten template for some time was garnished with a link to WorldCat at the price of weakening the understanding of what this was all about). Unfortunately MediaWiki at the moment has no provisions by which e.g. an independent Template:Worldcat id could drag neccessary parameters from another template like Template:Authority control. But it is commonly expected that the Wikidata project among other thins is suited to improve on this.
Connecting authority numbers found in Wikipedia articles with other data extracted from the same article (database links, interwiki links, categories, ...) can be used to yield even more BEACON files which give applications outside wikipedia access - although only partial since there are constrains by the two conditions that a corresponding wikipedia article has to exist and has to be equipped with at least some authority number - to resources normaly buried and scattered all over the encyclopedia. A PND to IMDb consisting of about 18.800 entries could be constructed by simply exploiting the appropriate IMDb_Name templates in en:WP and de:WP.
My personal understanding is that all these mappings derived from something imply a degradation of quality and obviously construction of further mappings by chaining does make things worse. But of course it is not feasible to compare any two matches from different resources intellectually and engage into discussions with the respective editors in case one detects errors and ambiguities: Authority data is about having a common frame of reference (at least within a certain community) and everybody doing its best to keep up the quality of mappings concerning his area of responsibility. Thus my engagement in de:wikipedia is twofold: Trying to keep up the quality of authority data by chasing for duplicates and formal errors and adding as many links as possible intellectually and contributing to the quality of the (german) authority file itself by engaging on the aforementioned feedback channel de:Wikipedia:PND/Fehlermeldung.